Foes of the Keystone XL oil pipeline appear to be winning the battle for endorsements from Hollywood celebrities and prominent personalities. Will it make a difference?

Actors Mark Ruffalo, Robert Redford, Kyra Sedgwick, Jared Leto, and Julia Louis-Dreyfus have come out against the $5.4 billion project. “Splash” star Daryl Hannah has been arrested at least three times at Keystone protests -- once after chaining herself to the White House fence. This week, former President Jimmy Carter joined nine other Nobel Peace Prize laureates in urging President Barack Obama to reject the project.

Having Hollywood stars and former heads of state on your side doesn’t assure success, but it can help “build up public concern and real outrage,” said Larry Noble, a counsel at the Campaign Legal Center, a Washington-based group that monitors campaign finance and other political issues.

“Those names will get someone to read something they might not have otherwise read,” he said in a phone interview.

As the Obama administration weighs the project, both sides fighting over the Canada-to-U.S. oil pipeline have sought to marshal current and former politicians, foreign dignitaries, scientists and celebrities to their cause.

The American Petroleum Institute, an oil and gas industry lobbying group, has said in advertisements that former presidents George W. Bush, a Republican, and Bill Clinton, a Democrat, support the pipeline. Canadian Prime Minister Stephen Harper has urged Obama to approve the project.

Critics say the carbon-heavy Canadian crude that Keystone would carry to refineries in the U.S. threatens to raise the risks of climate change.

Supporters say the oil will get to market even without the pipeline, which will provide construction jobs. They also discount the clout of celebrity endorsements, saying polls consistently show that Americans, by a wide margin, back the project.

“On one side you have people whose living is made from notoriety, and on the other you have supporters of KXL who have taken a stand because they believe it is the right thing to do for their labor union, or their constituents or the community,” said R.C. Hammond, a spokesman for the Consumer Energy Alliance, an industry-backed group whose members include Exxon Mobil Corp. (NYSE: XOM).

Asked to name celebrities on his side, Hammond said in an email that investors Warren Buffett and T. Boone Pickens favor Keystone, as does action-movie star Chuck Norris.

“It does matter,” said Noble, a former general counsel at the Federal Election Commission.

It’s particularly important for Keystone opponents to generate free publicity because they can’t match the money that the oil and gas industry has at its disposal, Noble said.

In an open letter to Obama, Carter and the other Nobel Peace Prize winners -- including anti-apartheid activist Archbishop Desmond Tutu -- call Keystone a “linchpin for tar sands expansion and the increased pollution that will follow.”

“History will reflect on this moment and it will be clear to our children and grandchildren if you made the right choice,” the Nobel winners wrote.

The letter was dated April 15 and published April 16 in Politico, a Washington-based publication. Former Vice President Al Gore, another Nobel winner, didn’t sign the letter although he has publicly opposed Keystone.

Anthony Swift, an international attorney at the Natural Resources Defense Council (NRDC), said Carter’s opinion in particular carries weight as a former president, “whose visionary energy policies have proved prescient in recent years.”

TransCanada Corp. (NYSE: TRP) of Calgary proposed Keystone in September 2008. The version under review now tracks an altered path across Nebraska in an effort to avoid concerns that the original route threatened a network of wetlands and an important aquifer underneath.

The State Department is reviewing whether the project is in the national interest, weighing factors including its environmental, economic and diplomatic impacts. The department has jurisdiction because Keystone would cross a U.S. border.

Eight federal agencies, including the Environmental Protection Agency, have until May 1 to submit their thoughts on Keystone. There’s no timeline for a final decision, which Obama, also Nobel Peace Prize winner, has said he will make after the State Department completes its review.

The NRDC, the Sierra Club, the Nobel Women’s Initiative and Environmental Defence, a Canadian environmental group, sponsored the Politico ad with the letter.

Shawn Howard, a spokesman for TransCanada, said the company “shares global concerns about the effects of climate change.” He said the environmental review, done as part of the State Department’s examination, found that Keystone wouldn’t have a major impact on greenhouse-gas emissions because the oil sands would be developed without the line.

Environmentalists say Keystone is crucial to oil sands production.